The present invention relates to mixtures of bis(silylorganyl)polysulfanes and silylorganylthiocyanates, as well as their preparation and use in rubber mixtures.
Trialkoxysilylpropylpolysulfanes are excellent bonding agents for the incorporation of oxide material into rubber matrices. Particularly in the tire industry, the bis(triethoxysilylpropyltetrasulfane (C.sub.2 H.sub.5 O).sub.3 Si(CH.sub.2).sub.3 !.sub.2 S.sub.4 ! is widely used in silica reinforced tires. There is a binding of the silane to the free hydroxyl groups of the silicic acid, on the one hand, and a vulcanization-like crosslinking with the rubber, on the other hand. For special uses, it is advantageous not to equip the silane with a tetrasulfane function but rather with a less reactive disulfane function. The synthesis of silylalkyldisulfanes is described together with their corresponding polysulfanes in various patents and publications.
German Patent Nos. 2,405,758 and 2,542,534 concern the preparation, proceeding from mercaptoalkyl silanes and sulfur, in which 1 hydrogen sulfide is released.
Several preparation methods are based on in-situ-produced disulfides, with which nucleophilic substitutions are then carried out on haloalkylsilanes. These methods differ only in the synthesis of the nucleophilic disulfide. According to German Patent No. 3,311,340, the disulfide is produced by reactions between hydrogen sulfide, sodium, and sulfur in ethanol.
According to U.S. Pat. No. 5,405,985, an aqueous sodium sulfide solution is used together with sulfur for the preparation. Persons skilled in the art are sufficiently aware that, during reactions between sulfides and sulfur, mixtures of various polysulfides are formed, so that during the nucleophilic substitution, in turn, only a mixture of polysulfanes with various chain lengths can result. The same is true for reactions between mercaptans or thiolates and sulfur. It is furthermore known that the corresponding disulfanes can be separated from these product mixtures only with great difficulty.
In German Patent No. 2,360,470, a method for the preparation of pure bis(silylalkyl)disulfane by the oxidation of the corresponding mercaptan with sulfuryl chloride is 20 described, but strongly corrosive by-products (SO.sub.2, HCl) are formed during this process. Secondary reactions on the silyl group also lead to a reduction of the yield of the desired product (example: 63.3%). Another oxidative variant is found in European Patent No. A1 217,178. Here, the corresponding thiolates are oxidized by iodine to form the disulfides. The process requires two more reaction steps after the expensive preparation of the silylalkylmercaptan.
In the unpublished German Patent application No. 195 41 404.7, a method for the selective synthesis of silylalkyldisulfanes is described.
In that document, polysulfanes are reacted for the desulfuration with nucleophilic reagents, such as NaCN. A disadvantage of this reaction, however, is to be found in the fact that per mol of the sulfur atoms to be removed, 1 mol of NaSCN is formed by the reaction with NaCN. This can be separated only in a cumbersome manner and then must be disposed of.
An object of the present invention is, above all, to avoid the inevitable yield of NaSCN that results from prior known processes.